1. Field of the Invention
The present invention generally relates to a projection apparatus, and more particularly, to a polarization conversion system and an illumination module having the polarization conversion system.
2. Description of Related Art
FIG. 1 is a diagram of a conventional illumination module. Referring to FIG. 1, an illumination module 100 includes a plurality of light sources 110, a lens array 120, a polarization conversion system 130 and a lens 140. The light sources 110 emit a plurality of non-polarized beams L1, and after the non-polarized beams L1 pass through the lens array 120 and are uniformed by the lens array 120, the non-polarized beams L1 are incident upon the polarization conversion system 130 and each of the non-polarized beams L1 is converted into a polarized beam L4 by the polarization conversion system 130. The lens 140 is disposed in the optical paths of the polarized beams L4.
The polarization conversion system 130 has a plurality of prisms 130a, a plurality of absorbing elements 132, a plurality of polarization beam splitting films (PBS films) 134, a plurality of reflection films 136 (also may be a plurality of polarization beam splitting films) and a plurality of half wave plates 138. The absorbing elements 132, the PBS films 134, the reflection films 136 and the half wave plates 138 are disposed on the prism 130a. As the non-polarized beams L1 are incident upon the polarization conversion system 130, the PBS films 134 respectively make each of the non-polarized beams L1 split into a first light beam L2 with a first polarization D1 and a second light beam L3 with a second polarization D2. The first light beams L2 respectively pass through one of the PBS films 134. The second light beams L2 are sequentially and respectively reflected by one of the PBS films 134 and reflected by one of the reflection films 136. Then, second light beams L2 pass through the half wave plates 138 and are respectively converted into another second light beam L3 with the first polarization D1. The first light beam L2 from the PBS film 134 and the second light beam L3 from the half wave plate 138 are merged into a polarized beam L4 with the first polarization D1.
If the non-polarized beams L1 are incident not upon the PBS films 134, a portion of the non-polarized beams L1 with the second polarization D2 may be emitted from the polarization conversion system 130, and the portion of the non-polarized beams L1 with the second polarization D2 are mixed up with the polarized beams L4, which would degrade the image quality of the projection apparatus using the illumination module 100. Therefore, the absorbing elements 132 are employed to block the non-polarized beams L1 from entering the polarization conversion system 130. However, the absorbing elements 132 prevent the non-polarized beams L1 from entering the polarization conversion system 130. In more detail, as the non-polarized beams L1 are incident upon the polarization conversion system 130, if the non-polarized beams L1 incident upon the lens array 120 are not parallel light beams, the non-polarized beams L1 would be incident upon the lens array 120 with a small incidence angle, which may enlarge the cross-section area of each of the non-polarized beams L1 after passing through the lens array 120 but prior to entering the polarization conversion system 130; thus, each of the non-polarized beams L1 is unable to completely propagate to the PBS film 134, so that a portion of the non-polarized beam L1 is absorbed by the absorbing element 132 resulting in an energy loss and a reduced optical performance of the light sources 110.